In typical optoelectronics applications light generated by a semiconductor laser is coupled to a glass (silica) optical fiber. However, the mismatch in optical mode dimensions between these two components (.about.1 .mu.m for the laser and .about.7 .mu.m for the fiber) makes direct coupling between them inefficient (e.g., only .about.10% of the light would be coupled into the fiber). Therefore, one or more lenses are typically introduced between the laser and the fiber to match the modes and improve the coupling efficiency to .about.50%. But this type of optical assembly requires tight alignment tolerances (.about.0.2 .mu.m). Achieving such tolerances is accomplished routinely in the optoelectronics industry, but the added complexity introduces significant cost to the end product. Most telecommunications applications can easily support the additional cost, but more commodity-oriented applications, such as data communication, cannot.
The prior art has attempted to address this problem by designing the semiconductor laser to include a monolithically integrated beam expander which performs the desired mode size conversion internal to the laser. This design eliminates the need for any bull optical components, such as lenses, between the laser and the fiber. In addition, alignment tolerances are increased by an order of magnitude to several micrometers, thereby significantly reducing packaging cost.
Expanded beam lasers also find application in conjunction with planar waveguide devices where a mode mismatch problem also exists. The use of an expanded beam laser reduces this problem and provides technologically obtainable laser component bonding tolerances of .about.2 .mu.m; i.e., the tolerance by which a laser component is positioned on and bonded to a submount (e.g., a silicon optical bench) relative to an optical waveguide or an optical fiber. The ability to mate active and passive components via expanded beam technology is one pathway to higher levels of component functionality. Increased functionality finds many applications such as wavelength selectable lasers, wavelength transponders and optical interconnects.
In the prior art beam expansion is achieved by judicious use of waveguide tapers. In a typical semiconductor laser the vertical waveguide dimension is much smaller than the wavelength of the laser light being confined by the waveguide. In this case thinning the vertical waveguide dimension results in increasing the vertical optical beam size. On the other hand, the lateral waveguide dimension of such a laser is on the order of the wavelength of the laser light. In this case beam expansion can be achieved either by widening the waveguide or by narrowing it. In many designs vertical and lateral (horizontal) tapers are combined to achieve the desired beam expansion in both vertical and horizontal directions. These tapers are gradual to ensure an adiabatic (low optical loss) transition of the optical mode from a relatively small size to a larger size.
Most expanded beam lasers reported in the prior art literature are constructed in a serial fashion; i.e., the laser section is followed by a passive waveguide section. This design is dictated by the need for a strong overlap of the optical beam with the gain (active) medium. One of the principal difficulties in realizing a practical expanded beam laser lies in monolithically integrating a standard laser section, which generates the laser light, with some form of tapered waveguide section which expands the beam without introducing significant optical loss. Illustrative of the prior art literature are the following articles all of which are incorporated herein by reference: R. Ben-Michael et al., IEEE Photon. Tech. Lett., Vol. 6, No. 12, pp. 1412-1414 (1994), I. Moerman, IEEE J. Selected Topics in Quantum Electron., Vol. 3, No. 6, pp. 1308-1320 (1997), and R. Ben-Michael et al., U.S. Pat. Nos. 5,574,742 and 5,720,893 issued on Nov. 12, 1996 and Feb. 24, 1998, respectively.
Both the taper formation and the concatenation of the two sections of a monolithically integrated expanded beam laser involves technology that is not available in present manufacturing processes for optoelectronic devices. Thus, a need remains in the art for a reliable fabrication process that is capable of producing expanded beam devices without introducing significant optical loss or significant manufacturing cost.